Replace eframe::Frame commands and WindowInfo with egui (#3564)

* Part of https://github.com/emilk/egui/issues/3556

## In short
You now almost never need to use `eframe::Frame` - instead use
`ui.input(|i| i.viewport())` for information about the current viewport
(native window), and use `ctx.send_viewport_cmd` to modify it.

## In detail

This PR removes most commands from `eframe::Frame`, and replaces them
with `ViewportCommand`.
So `frame.close()` becomes
`ctx.send_viewport_cmd(ViewportCommand::Close)`, etc.

`frame.info().window_info` is now also gone, replaced with `ui.input(|i|
i.viewport())`.

`frame.info().native_pixels_per_point` is replaced with `ui.input(|i|
i.raw.native_pixels_per_point)`.

`RawInput` now contains one `ViewportInfo` for each viewport.

Screenshots are taken with
`ctx.send_viewport_cmd(ViewportCommand::Screenshots)` and are returned
in `egui::Event` which you can check with:

``` ust
ui.input(|i| {
    for event in &i.raw.events {
        if let egui::Event::Screenshot { viewport_id, image } = event {
            // handle it here
        }
    }
});
```

### Motivation
You no longer need to pass around the `&eframe::Frame` everywhere.
This also opens the door for other integrations to use the same API of
`ViewportCommand`s.

examples/custom_window_frame/src/main.rs

@@ -2,7 +2,7 @@
 
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 
-use eframe::egui;
+use eframe::egui::{self, ViewportCommand};
 
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
@@ -30,8 +30,8 @@ impl eframe::App for MyApp {
         egui::Rgba::TRANSPARENT.to_array() // Make sure we don't paint anything behind the rounded corners
     }
 
-    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
-        custom_window_frame(ctx, frame, "egui with custom frame", |ui| {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        custom_window_frame(ctx, "egui with custom frame", |ui| {
             ui.label("This is just the contents of the window.");
             ui.horizontal(|ui| {
                 ui.label("egui theme:");
@@ -41,12 +41,7 @@ impl eframe::App for MyApp {
     }
 }
 
-fn custom_window_frame(
-    ctx: &egui::Context,
-    frame: &mut eframe::Frame,
-    title: &str,
-    add_contents: impl FnOnce(&mut egui::Ui),
-) {
+fn custom_window_frame(ctx: &egui::Context, title: &str, add_contents: impl FnOnce(&mut egui::Ui)) {
     use egui::*;
 
     let panel_frame = egui::Frame {
@@ -66,7 +61,7 @@ fn custom_window_frame(
             rect.max.y = rect.min.y + title_bar_height;
             rect
         };
-        title_bar_ui(ui, frame, title_bar_rect, title);
+        title_bar_ui(ui, title_bar_rect, title);
 
         // Add the contents:
         let content_rect = {
@@ -80,12 +75,7 @@ fn custom_window_frame(
     });
 }
 
-fn title_bar_ui(
-    ui: &mut egui::Ui,
-    frame: &mut eframe::Frame,
-    title_bar_rect: eframe::epaint::Rect,
-    title: &str,
-) {
+fn title_bar_ui(ui: &mut egui::Ui, title_bar_rect: eframe::epaint::Rect, title: &str) {
     use egui::*;
 
     let painter = ui.painter();
@@ -112,9 +102,11 @@ fn title_bar_ui(
 
     // Interact with the title bar (drag to move window):
     if title_bar_response.double_clicked() {
-        frame.set_maximized(!frame.info().window_info.maximized);
+        let is_maximized = ui.input(|i| i.viewport().maximized.unwrap_or(false));
+        ui.ctx()
+            .send_viewport_cmd(ViewportCommand::Maximized(!is_maximized));
     } else if title_bar_response.is_pointer_button_down_on() {
-        frame.drag_window();
+        ui.ctx().send_viewport_cmd(ViewportCommand::StartDrag);
     }
 
     ui.allocate_ui_at_rect(title_bar_rect, |ui| {
@@ -122,13 +114,13 @@ fn title_bar_ui(
             ui.spacing_mut().item_spacing.x = 0.0;
             ui.visuals_mut().button_frame = false;
             ui.add_space(8.0);
-            close_maximize_minimize(ui, frame);
+            close_maximize_minimize(ui);
         });
     });
 }
 
 /// Show some close/maximize/minimize buttons for the native window.
-fn close_maximize_minimize(ui: &mut egui::Ui, frame: &mut eframe::Frame) {
+fn close_maximize_minimize(ui: &mut egui::Ui) {
     use egui::{Button, RichText};
 
     let button_height = 12.0;
@@ -137,22 +129,24 @@ fn close_maximize_minimize(ui: &mut egui::Ui, frame: &mut eframe::Frame) {
         .add(Button::new(RichText::new("❌").size(button_height)))
         .on_hover_text("Close the window");
     if close_response.clicked() {
-        frame.close();
+        ui.ctx().send_viewport_cmd(egui::ViewportCommand::Close);
     }
 
-    if frame.info().window_info.maximized {
+    let is_maximized = ui.input(|i| i.viewport().maximized.unwrap_or(false));
+    if is_maximized {
         let maximized_response = ui
             .add(Button::new(RichText::new("🗗").size(button_height)))
             .on_hover_text("Restore window");
         if maximized_response.clicked() {
-            frame.set_maximized(false);
+            ui.ctx()
+                .send_viewport_cmd(ViewportCommand::Maximized(false));
         }
     } else {
         let maximized_response = ui
             .add(Button::new(RichText::new("🗗").size(button_height)))
             .on_hover_text("Maximize window");
         if maximized_response.clicked() {
-            frame.set_maximized(true);
+            ui.ctx().send_viewport_cmd(ViewportCommand::Maximized(true));
         }
     }
 
@@ -160,6 +154,6 @@ fn close_maximize_minimize(ui: &mut egui::Ui, frame: &mut eframe::Frame) {
         .add(Button::new(RichText::new("🗕").size(button_height)))
         .on_hover_text("Minimize the window");
     if minimized_response.clicked() {
-        frame.set_minimized(true);
+        ui.ctx().send_viewport_cmd(ViewportCommand::Minimized(true));
     }
 }